Organic electroluminescent (organic EL) light-emitting apparatuses have superior characteristics such as less viewing angle dependence, low power consumption and a high realizability of a very thin structure, and have newly drawn public attentions as light-emitting apparatuses due to increasing demands for flat displays. In an attempt to practically use such organic EL light-emitting apparatuses, one of problems is that the light-emitting life thereof is short. In order to solve this problem, heretofore, the improvements of the light-emitting layer materials and the like of an organic EL device to be installed in organic EL light-emitting apparatuses have been vigorously carried out.
On the other hand, in order to improve the light-emitting life, an attempt has been made to improve the external energy efficiency of the organic EL device. It has been known that the external energy efficiency is represented by the product of the internal energy efficiency and light extraction efficiency of a device. That is, in order to improve the external energy efficiency of the organic EL device, there is a method in which in addition to the improvement of the internal energy efficiency, the light extraction efficiency is also improved.
The light extraction efficiency refers to the rate of the quantity of light released into the atmosphere from the front side of an organic EL light-emitting apparatus to the quantity of light emitted from the light-emitting layer of an organic EL device. When light emitted from the light-emitting layer is released into the atmosphere, the light needs to pass through some interfaces, each formed between media having different refractive indexes, and based upon Snell's law, light that is made incident on each interface with an angle that is greater than its critical angle, is totally reflected by the interface to progress into the layer to disappear, or is released from the side faces of the layer, with a result that the quantity of light to be released from the front side of an organic EL light-emitting apparatus is reduced by the corresponding portion. The light extraction efficiency of the light-emitting layer of a conventional organic EL device is about 18%, with a result that about 82% of light generated in the light-emitting layer is confined inside the light-emitting apparatus to disappear, or is released from the side faces of the light-emitting apparatus.
For this reason, it is an important problem to improve the light extraction efficiency, and various attempts have been made. For example, a substrate has been proposed in which one of the surfaces of a glass substrate is rubbed with an abrasive material or the like to be roughened, and this is used as a transparent substrate so that light emission is scattered (Patent Document 1: Japanese Patent Laid-open Publication No. 61-156691), and a substrate has been proposed in which a light-emitting layer is sandwiched with a pair of transparent electrode layers, with a scattering region being provided in the vicinity of the interface between the transparent electrode layer and the light-emitting layer (Patent Document 2: Japanese Patent Laid-open Publication No. 09-129375).